1. Field of the Invention
The present invention relates to a precision feeding device and precision transfer equipment, more specifically, to a device that transfers a work table or a column of measuring equipment, a machine tool or the like with a high precision for positioning.
2. Description of Related Art
In precision transfer equipment requiring a high motion accuracy (e.g., a precision positioning system, precision measuring equipment and a machine tool), an object to be moved (e.g. a work table, spindle column) is required to be moved with a high geometrical motion accuracy and, in addition, a position in a linear moving direction has to be accurately detected and controlled.
In detecting and controlling the position of the object to be moved, it is desirable that the precision transfer equipment mechanically satisfies the Abbe's principle. Specifically, it is desirable that a detection axis for identifying the position of the object passes through a point of action for a measurement or processing performed by moving the object. In addition, it is desirable that both a drive axis (or thrust axis) and a guide axis pass through a center of gravity of the object in order to control the movement and positioning of the object, and these drive (or thrust) and guidance axes coincide with the detection axis.
In a conventional drive mechanism of precision transfer equipment, a transmission mechanism such as a ball screw, friction drive, belt drive and rack-and-pinion is used (see Patent Literature 1: JP-A-2006-205292 and Patent Literature 2: JP-A-2000-55617).
In the equipment disclosed in Patent Literature 1, a transmission mechanism in a form of a drive roller is used in driving an object to be moved (e.g. a table) by a drive mechanism, in which the drive roller is frictionally rolled on a rod (driving body) to apply a drive force. In this device, a follower roller that receives a pressing force of the drive roller is disposed on an opposite side of the rod. However, the drive force is applied only on the side of the drive roller.
In the equipment disclosed in Patent Literature 1, a fluid static pressure joint is used for connecting the rod and the table. Especially, a supplying-discharging static-pressure bearing or a vacuum-balanced static-pressure air bearing can provide a strong rigidity in a compression direction and a tensile direction. Thus, there has been proposed to use a drive mechanism using the static-pressure bearing having such a high rigidity to increase geometric accuracy of motion of a high precision linear moving table.
The static-pressure joint disclosed in Patent Literature 1 includes a thrust plate that is supported near the rod by a gimbal mechanism absorbing a motion error in a yawing direction and is supported near the table by a similar gimbal mechanism absorbing a motion error in a pitching direction. Through the joint, forces other than the force in the moving direction of the table are not transmitted.
In a device according to Patent Literature 2, a slider driving mechanism using a feed screw shaft is provided under a table and the table is horizontally moved by the feed screw shaft.
A position detecting mechanism using a laser interferometer is provided on a side of the table of the device of Patent Literature 2. In this arrangement, a laser path extending from a light source of the laser interferometer to an end of the table is surrounded by a stretchable tube with a bellows, in which an inner pressure of the tube is reduced to eliminate an influence from air turbulence in the laser path, thereby achieving further high precision.
With the use of the laser interferometer, it is possible to acquire a high geometric accuracy of motion of the drive mechanism and the movable body and accurately detect and control the position of the movable body in a feed direction.
However, the above-described Patent Literatures 1 and 2 respectively use a frictional drive roller and a ball feed screw drive, where a moving axis (i.e. a direction for a moving component of the drive mechanism to be guided) and a thrust axis (i.e. a direction in which a drive force is applied) of the drive mechanism do not necessarily coincide and consequently an offset arises.
On the other hand, it is extremely difficult in terms of design to coincide the moving axis and thrust axis and pass through a gravity center of the object, to dispose the detection axis so that the detection axis passes through a point of action for measurement or processing, and to coincide all of the moving axis, thrust axis and detection axis.
If Abbe's principle is not satisfied and an influence of an offset is not negligible in such a device, supposing that the positioning of the drive mechanism has high repeatability, an additional step for calibration or correction using the third independent position measuring instrument (e.g. length measuring instrument with laser interferometer) that serves as an upper standard has to be conducted for an action point of a measurement and processing performed by driving the object to be moved, thus causing complication of device structure, decrease in serviceability and increase in production cost.
The above occurs not only in precision transfer equipment including a movable body such as a table but also in a device for driving the movable body (i.e. precision feeding device that does not include an object to be moved).